WHITEFISH OF THE LAKE OF GENEVA 



for their subsistence in these creatures. In the cycle 

 of nutrition, the plankton is the foundation, and the 

 rest follows from it. According to its annual alterna- 

 tions, its phases of prosperity or diminution, the fish 

 population, maintained and nourished by it, increases 

 or decreases, each species finding its own manner 

 of life and its own particular principle of existence 

 therein. 



Mediaeval medicine had a word, now no longer 

 used, which expressed the capacity of an organism for 

 alimentation and assimilation. It was taken directly 

 from the Greek word " sitesis ". It meant the power 

 which a creature possesses within itself, to exist by 

 feeding itself, and the variable rate of that power in 

 its equilibrium of complete existence. To avoid 

 periphrasis we may perhaps revive the word here. 

 The lake, as regards the food-supplying capacities of 

 its water, has its ratio of sitesis, its definite capacity 

 of providing nourishment. This determines the size 

 of its population, and keeps it within the bounds 

 it fixes. Its capacities are not indeterminate or 

 unlimited. 



What is true of the lake is true of other stretches 

 of water of the same type, and of the streams which 

 enter it. Each, like it, makes up a realm with an 

 alimentary capacity of its own, its sitesis, on which 

 depend the abundance and diversity of the species 

 which dwell in it. Some base it entirely on the tiny float- 

 ing creatures: others add to it the further resources 

 provided by the shores and land, and the larvae of insects 

 or midges and the like, which fall into the water. But 

 they all follow an identical principle, that of the chain 

 of devourers and devoured, of the equilibrium of 

 alimentation, of the mutual dependence of all, down to 

 the tiny creatures who form the basis of a vast edifice 

 of nutrition. A stream rich in fish is one whose 

 waters contain a store of nourishment sufficiently 

 abundant for them; and we cannot get much from a 

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